Para-xylene is a commodity which can be retreived from C.sub.8 aromatic mixtures containing the same. The C.sub.8 aromatic mixtures include ethyl benzene and the xylene isomers ortho-xylene, meta-xylene and para-xylene. The patent literture describes various processes for increasing the para-xylene content of such C.sub.8 aromatic mixtures, for example, by xylene isomerization processes.
Para-xylene is a raw material, supplied in C.sub.8 aromatic mixtures from naphtha-crackers or the disproportionation of toluene, as well as by reforming processes.
Para-xylene is the most valuable commodity of the C.sub.8 aromatic mixture fractions because it is a direct precursor in the production of terephthalic acid and dimethylterephthalic, each of which can be used to produce polyester, polyester fiber and films.
Ethyl benzene, the intermediate in synthesis of styrene, is a valuable commodity and is often contained in the C.sub.8 aromatic mixtures. Recovery of ethyl benzene is desirable but extremely difficult. As a result, ethyl benzene is often converted to benzene, toluene and xylenes at high severity conditions.
One of the problems which inheres in the production of para-xylene is its isolation from a typical C.sub.8 aromatic mixture. Prior art processes for separating para-xylene from a C.sub.8 aromatic mixture, also containing ortho-xylene, meta-xylene and ethyl benzene have been described. Those processes are cumbersome, because of the physical properties of the components of the C.sub.8 aromatic mixtures are very similar. For example, ortho-xylene, has a boiling point 3.5.degree. C. higher than tht of its closest boiling C.sub.8 aromatic isomer (meta-xylene); conventional fractional distillation techniques to separate ortho-xylene from meta-xylene include fractional distillation towers which contain 100 to 150 trays and operate at about a 5-8 to 1 reflux ratio. Ethyl benzene can be separated from such mixtures, but because its boiling point is 2.degree. C. different from the boiling point of para-xylene, the typical ethyl benzene fractionator contains 300-400 actual trays and requires about a 25-50 to 1 reflux to feed ratio. Since the meta- and para-xylene differ by only 0.7.degree. C. in boiling point, separation of these isomers by distillation is virtually impractical.